in file Twitter.php, method Services_Twitter::__get() Get interceptor, if the requested property is "options", it just return the options array, otherwise, if the property matches a valid API category it return an instance of this class.
in file Twitter.php, variable Services_Twitter::$api The Twitter API mapping array, used internally to retrieve infos about the API categories, endpoints, parameters, etc...
in file Twitter.php, variable Services_Twitter::$request The HTTP_Request2 instance, you can customize the request if you want to (proxy, auth etc...) with the get/setRequest() methods.